Satellites or spacecraft flying in circular orbit at approximately 36,000 km above the equator seem to be almost stationary to ground observers because their period of orbital motion and the rotation period of the earth are nearly equal. Such satellites are called geostationary satellites. However, in fact, the geostationary satellites are subject to various perturbative forces such as tidal force from the earth and the sun, perturbative force due to non-uniform gravity potentials of the earth, and solar radiation pressure; consequently, their latitude and longitude gradually change. Therefore, the satellites have to fire the thrusters for correcting change in the latitude and longitude. The thruster firing for correcting change in the longitude is generally called east-west control and the control for correcting change in the latitude is generally called south-north control. Performing the east-west control and the south-north control to keep the longitude and the latitude of a satellite in desired ranges is generally called orbital station-keeping.
A technique for realizing the orbital station-keeping of a geostationary satellite is described in, for example, Non-Patent Literature 1. The technique in the Non-Patent Literature 1 performs orbital station-keeping control on a geostationary satellite where electric propulsion devices are disposed in a petal-like pattern using a nonlinear optimum control technique. The Non-Patent Literature 1 fires four thrusters simultaneously and has an orbital station-keeping accuracy of approximately 0.005 degrees.